DE3878480T2 - LIQUID CRYSTAL DISPLAY DEVICE. - Google Patents

Description

This invention relates to a liquid crystal display device comprising a pair of spaced apart carrier plates, with liquid crystal material between the plates, with a number of picture elements in a matrix field defined by respective opposing electrodes on the carrier plates, and with a number of switching elements on one of the carrier plates and in series between address lines on this plate and the picture elements.

A liquid crystal display device of this type is suitable for displaying alphanumeric or video information.

In one known form of such a display arrangement, thin film transistors (TFTs) are used as switching elements and are located next to respective picture element electrodes at the interface of groups of column and row address lines on one carrier plate.

In another form of such a display device, a simplified structure is obtained using non-linear two-terminal elements as switching elements. These elements can be in the form of diode structures, for example as described in US Patent Specification 4,223,308, or as described in British Patent Specification 2,091,468, in which the diode structures have a metal-insulator-metal layer sequence. In these devices, the picture elements are arranged in cells and columns, with one of the carrier plates carrying rows of address lines and the other plate carrying columns of address lines, each picture element being connected to a two-terminal switching element between individual row and column address lines.

In these known forms of liquid crystal display devices, the switching elements and the associated address lines are usually arranged sideways to their associated picture element electrodes on one carrier plate and are connected to the picture element electrodes by means of extensions formed integrally with the electrodes. Accordingly, with regard to the available area of the display device, a certain part of the available area is allocated to the switching elements and the associated address lines, and the pixel electrodes are restricted to the remaining available area. In order to maximize the proportion of the available area occupied by the actual pixels, it was necessary to keep the physical dimensions of the switching elements and the associated address lines to a minimum.

One form of matrix liquid crystal display device has been proposed in EP-A-0182484 in which the picture element electrodes overlie their associated address lines on a support plate and a continuous layer of non-linear material is interposed so that each switching element is located below its associated element electrode. EP-A-0011408 and EP-A-0184341 similarly describe display device structures in which address lines extend below the picture element electrodes on a support plate with a two-terminal switching element located between each picture element electrode and its underlying address line, but relate only to display devices operable only in the reflective mode.

It had long been recognized that the type of active matrix address display devices were difficult to manufacture with sufficient reliability over large areas, and yield problems were common. Failure of individual switching elements causes pixel failures and can result in entire rows or columns of pixels becoming defective, thus rendering the device unusable. In an attempt to overcome this problem in TFT display devices, it has been proposed to allocate more than one thin film transistor to each pixel for redundancy purposes. However, adding additional switching elements increases the ratio of non-effective display production areas to effective display production area.

The invention is based on the object of creating an active matrix-addressed liquid crystal display device in which the ratio between ineffective and effective display production areas can be set to a minimum.

It is a further object of the invention to provide an active matrix-addressed liquid crystal display device whose degree of fault tolerance is such that it is less adversely affected by the effects of addressing element errors.

The invention is further based on the object of creating an active matrix-addressed liquid crystal display arrangement which is suitable for comparatively simple manufacturing processes and can be manufactured economically.

According to the invention, a liquid crystal display device is provided with a pair of spaced-apart carrier plates, with a liquid crystal material between the plates, a number of picture elements in a matrix structure which are defined by mutually opposite electrodes on the carrier plates, and with a number of switching elements on one of the carrier plates and in series between address lines on this plate and the picture elements, in which the picture element electrodes on one carrier plate overlie both the switching elements and the associated address lines on this one carrier plate, the switching elements being arranged between these picture element electrodes and address lines, characterized in that each of the picture element electrodes on the one carrier plate contains a number of discrete sub-electrodes, each of which is associated with at least one switching element, the switching elements associated with a picture element electrode being connected in parallel to one another to the corresponding address line.

In addition to avoiding the need to occupy areas on one carrier plate for mounting the laterally arranged switching elements and address lines by using a vertical structure and thus bringing about a more efficient use of the carrier plate area for display production purposes, the division of each picture element electrode into sub-electrodes, each of which is associated with at least one switching element, results in a particularly advantageous fault tolerance.

Each picture element is in fact divided into a number of sub-elements, each of which is individually controllable via one or more of its associated switching elements. Thus, when a display signal is applied to the picture elements, the individual picture sub-elements of each of the picture elements are switched independently of one another via their respective switching elements to produce a display effect from the picture element. An acceptable display effect can still be obtained even if one or more of the picture sub-elements is not operational, depending on the number of picture sub-elements and their dimensions with respect to the picture element itself. Although the display signal produced by a picture element with some defective picture sub-elements display effect is inevitably affected to some extent, this is not apparent to an observer looking at a device having a large number of picture elements, typically hundreds of thousands, each occupying an area of only, say, 300 by 300 µm. The device can therefore still be used with satisfactory results with a number of picture elements affected in this way, unlike known devices which use a single switching element for each entire picture element, and in which failure of one switching element results in complete failure of at least the associated picture element, and perhaps renders the device unusable.

Since a vertical structure is used, each of the address lines on one of the carrier plates can have a width up to the width of the picture element. In practice, the transverse dimension of the address lines can therefore be substantially equal to that of the picture element and the transverse dimension of the address lines used to determine the corresponding dimension of the picture elements.

The display device can be designed to operate in the reflection mode. However, in a preferred embodiment, the device is designed to operate in the transmission mode. In this case, and so that the address lines on one carrier plate do not substantially prevent the passage of light through the picture elements, these address lines are made substantially transparent, for example with indium tin oxide (ITO).

For simplicity and ease of handling, the switching elements are preferably non-linear two-pole, bidirectional devices such as diode structures. Considering that such diode structures, for example metal-insulator-metal layers, can be formed with a very small dimension relative to the picture element, their presence under the picture subelements defined by the subelectrodes need not significantly affect the passage of light through the display device when operating in the transmission mode. To avoid the possibility of light transmission obstruction and to obtain optimum transmission properties, the switching elements can be formed so that they have substantial transparency to light. For example, the elements can comprise a substantially transparent MIM-type diode structure with a thin film of silicon nitride sandwiched between two ITO layers. It is not necessary to change the physical dimensions of the switching elements to minimize the area they occupy. The number of transparent switching elements that can be provided for each image sub-element is not limited by light transmission considerations, but only by the relative physical dimensions of the switching elements and the image sub-elements.

As in conventional matrix liquid crystal display devices, the picture element electrodes on the one support plate are preferably generally planar and extend in a plane substantially parallel to the facing surface of the support plate and therefore in this case the number of switching elements associated with each picture element can be suitably arranged substantially in a planar region beneath the picture element electrode. The array of switching elements associated with each picture element can occupy an area corresponding to at least a major part of the area of the picture element electrode and preferably the array is such that the switching elements of the array are arranged substantially uniformly and spaced apart with respect to the area of the picture element electrodes. In other words the number of switching elements for each picture element can be scattered to occupy individual and regularly spaced apart discrete areas in an area beneath the picture element electrode substantially corresponding to the area of the picture element.

The use of diode structures as two-pole bidirectional switching elements is particularly advantageous and offers further advantages. In addition to MIM-type diode structures of this type with silicon nitride, for example in the form of non-stoichiometric silicon nitride, as the insulator component sandwiched between ITO layers, silicon oxide may also be used instead of silicon nitride. Another form of MIM-type element that can be used may use an anodized tantalum film with an overlying conductive layer, which does not necessarily have to be transparent. In one embodiment of the invention, a deposited conductive layer, for example of ITO, and in the function of one of the end layers of the diode structure may also serve as the picture element electrode. Thus, the picture element electrode is formed integrally with one of the conductive end parts of the diode structure as a unitary layer. The other conductive end component of the diode structure may suitably be formed by a respective part of the associated address line on one carrier plate. wherein the first-mentioned conductive end layer is arranged through the layer of insulator vertically to the carrier plate at a distance from this address line.

In another embodiment in which two-pole switching elements are used in the same way, the pole of the elements facing away from one of the carrier plates is arranged at a distance from its associated picture element sub-electrode by insulating material, which insulating material is for example silicon nitride to form a capacitive coupling. As before, the other pole of the switching element is formed by a respective part of the address line on the one carrier plate. The first-mentioned poles of the switching element can be formed by conductive material grains, for example ITO distributed in an insulator matrix material, which are arranged at a distance from the other conductive poles of the elements by a thin insulating film and thus form a metal-insulator-metal layer sequence. This distribution can be a random distribution, but with sufficient density to ensure that each picture element sub-electrode is assigned by capacitive coupling to at least one switching element formed in this way and preferably to a number of switching elements. This type of device formation avoids the use of conventional photolithographic processes and can be carried out using simpler techniques, for example by using powdered ITO and by scattering the ITO grains over the thin insulator layer that lies on the address lines on one carrier plate.

In a variation of this embodiment, the poles of the switching elements located at a distance from one of the carrier plates are formed as discrete points of conductive material, for example ITO, which have been deposited over a mask, so that the positioning and spatial distance between the switching elements formed in this way are largely a predetermined distribution rather than a random distribution. This deposition can involve evaporation of the conductive material through a suitable perforated mask. In addition to avoiding the need for a photolithographic process, this technique offers the further advantages that it is relatively inexpensive and, compared to the aforementioned random distribution technique, the number of diode structures for each pixel and, more precisely, the number of diode structures, whether there is only one or several diode structures and assigned to each pixel sub-electrode, is controllable.

In any of the above embodiments or modifications the display arrangement may include storage capacitors associated with the picture elements. Each picture sub-element is preferably associated with a respective storage capacitor. These storage capacitors may be partially formed by at least one conductive layer on the one carrier plate, which underlies the picture element sub-electrodes on the one carrier plate and is electrically insulated therefrom. In addition to partially forming the storage capacitors, the one or more conductive layers have a further and advantageous function in that they are arranged between the picture element sub-electrodes from the underlying address lines and serve to shield the sub-electrodes from their underlying address lines. This shielding minimizes any capacitance between a picture element and its address line.

Several embodiments of liquid crystal display devices according to the invention are explained in more detail below with reference to the drawing. They show

Fig. 1 shows in flat schematic form a part of an active matrix liquid crystal display device according to the invention with a matrix field of picture elements controlled by non-linear two-terminal switching arrangements,

Fig. 2 is a schematic cross-section, not to scale, through an embodiment of the display arrangement according to the invention,

Fig. 3 is a broken-away schematic plan view of part of the display arrangement according to Fig. 2,

Fig. 4 shows schematically the effective electrical circuit of a typical picture element of the picture elements of the arrangement according to Fig. 2,

Fig. 5 is a schematic cross-section (not to scale) through another embodiment of a display arrangement according to the invention,

Fig. 6 is a schematic cross-section, not to scale, through a further embodiment of a display arrangement according to the invention,

Fig. 7 is a schematic representation of the effective electrical circuit of a typical pixel of the pixels of the arrangements according to Figs. 5 and 6,

Fig. 8 and 10 modifications of the display arrangements according to Fig. 2 and 5, respectively, again not to scale, in which storage capacitors are provided for the picture elements,

Fig. 9 and 11 are schematic operative circuit diagrams of typical examples of the picture elements of the arrangements according to Fig. 8 and 10 respectively, which are similar with respect to the circuits in Fig. 4 and 7 respectively, and

Fig. 12 is a schematic cross-section, not to scale, through another display arrangement according to the invention, also with storage capacitors for the picture elements, and in which the effective circuit of a typical picture element is generally the same as that according to Fig. 11.

As is true for all the various embodiments and modifications described below, Fig. 1 shows part of an active matrix liquid crystal display device having a plurality of picture elements 10 in a matrix array of rows and columns and containing spaced apart electrodes with liquid crystal material between them. Only six picture elements are shown in Fig. 1. The picture elements 10 are addressed via two groups of address lines in the form of conductors 12 and 14 on the facing surfaces of the two glass support plates (not shown in Fig. 1) on which the picture element electrodes are also located. The conductors of each group are arranged parallel to each other and the two groups extend perpendicular to each other in their spaced apart planes and define the picture element areas at their intersection points.

The row conductors 12 serve as scanning electrodes and are driven by a driver circuit (not shown) which applies a scanning signal to each row electrode 12 in sequence. In synchronism with these scanning signals, data signals are applied to the columns of conductors 14 to produce the required display from the rows of picture elements associated with the row conductors 12 when scanned. In a video or television display arrangement, these data signals comprise video information signals. By suitably selecting a sufficiently large difference between the scanning and data signals, the optical transmittance of a selected picture element at the interface of a row conductor 12 and a column conductor 14 can be varied to produce a visible display effect. The picture elements are activated only to produce a display effect in response to both the scanning signal and the data signal applied by the switching elements 16 in the form of non-linear elements in series between each picture element 10 and its row conductor 12. The signal applied to a row conductor 12 The scanning signal causes the switching elements associated with the pixels of this line to conduct and the scanning signal to be passed on to the pixel. In conjunction with the data signal, the scanning signal produces the desired optical effect from the pixel. The data signal itself is insufficient to trigger such an effect. The individual display effects of the large number of pixels, each addressed line by line, are combined to build up a complete display. By utilizing the transmission/voltage characteristics of a liquid crystal pixel, grayscale levels can be achieved.

The voltage/conductance characteristic of the switching element is ideally bidirectional and symmetrical with respect to zero voltage, so that a net DC bias across the picture elements and consequent electrochemical degradation of the liquid crystal material is avoided. However, a voltage/conductance characteristic close to the ideal case is acceptable. For simplicity, the polarity of the drive voltages, i.e. the scan and data signals, is reversed after each complete field.

Active matrix liquid crystal display devices having non-linear two-terminal switching elements in series with and laterally spaced from the picture elements are well known. The main elements and general operation of the display device of Figure 1 are in certain respects similar to those of these known devices and accordingly the above description with reference to Figure 1 is intentionally brief. For further information on these features, reference is therefore made to earlier publications describing these generally similar types of display devices, such as US Patent Specification 4,223,308 and British Patent Specification 2,147,135, both of which describe the use of diodes as switching elements, and British Patent Specification 2,091,468, which describe the use of metal-insulator-metal layer sequences as switching elements, parts of the descriptions of which may be considered to form part of this description. Other forms of non-linear switching elements are usable in this invention, for example p+-i-p+, n+-p-n+ diode structures.

Several embodiments of display arrangements and their modifications according to the invention are described below. In each of these embodiments, the display elements mounted on one of the carrier plates Picture element electrodes are each associated with a number of separately operating switching elements for fault tolerance purposes, which are arranged between these picture element electrodes and the associated address line on the carrier plate and are subject to the area occupied by the picture element electrodes. Furthermore, each picture element electrode is divided into a number of discrete sub-electrodes, which are each connected to at least one respective element of the number of switching elements associated with this picture element, so that voltages applied to these sub-electrodes are controlled by the respective switching elements. Although the liquid crystal display device according to the invention can be operated in the reflection mode, all of the following embodiments are designed to operate in the transmission mode. In view of this, the address lines on the aforementioned carrier plate are designed to be substantially transparent in order not to significantly impede the passage of light through the picture elements. Since their dimensions are not physically significant relative to the pixel dimensions, the switching elements are also substantially transparent and in these particular embodiments comprise diode structures operating as MIM thin film structures utilizing the Poole-Frenkel effect. As examples of typical dimensions, the pixels may be about 300 µm² and if each pixel contains, say, nine sub-electrodes each associated with a single switching element, each sub-electrode may be about 100 by 100 µm² and each switching element about 5 to 10 µm².

For simplicity, the same reference numerals are used to designate corresponding parts of the various embodiments.

Fig. 2 shows a schematic cross section through a first embodiment of an active matrix display arrangement according to the invention. The arrangement contains two transparent carrier plates 20 and 22 which are spaced apart from one another and whose surfaces facing one another are at least insulating and between which liquid crystal material 24 is arranged. The plates can be made of any suitable material, for example glass.

The upper support plate 20 carries the group of column conductors 14, one of which is visible in Fig. 2 and is made of transparent conductive material such as indium tin oxide (ITO). The conductors 14 and intermediate areas of the plate surface are covered by an insulating alignment layer 25 of polyimide.

The lower support plate 22 carries the group of row conductors 12, two of which are shown in Fig. 2, which in turn are formed from transparent conductive material such as ITO. The two groups of conductors 12 and 14, as already mentioned, extend essentially perpendicular to one another and determine at their intersections the locations of the matrix of picture elements, which are designated here by 10. The conductors 12 and 14 shown are each formed as planar strips of essentially constant width, thereby producing essentially rectangular picture elements. Alternatively, the conductors 12 and 14 need not be formed as unitary strips, but can contain discrete fields of conductive material in the picture element areas, which are connected to one another by separately formed conductive tracks.

An insulating layer 27 is applied to the conductors 12 and the intermediate surface areas of the plate 22, which is deposited either as a single layer or as a plurality of separate layers of silicon nitride, although silicon dioxide may be used instead. Portions of this silicon nitride layer 27 in the areas where the conductors 12 and 14 intersect and in areas substantially corresponding to the individual pixel areas defined by the area of overlap between the conductors 12 and 14 at their interfaces are formed with pits 30 by selective photolithographic etching through a mask. Each such portion of layer 27 comprises a matrix of regularly spaced pits, i.e. a 4-by-4 matrix in the example of Figure 2, although only four such pits are visible in the cross-section shown, corresponding to an area substantially corresponding to the pixel area on conductors 12 and substantially uniformly spaced from one another with respect to that area.

After etching these pits 30, a thin film, about 150 angstroms, of non-stoichiometric silicon nitride is deposited on the structure, thereby forming a thin insulating film on the surface of conductors 12 at 32 on the bottom of each pit.

Discrete dots 34 of transparent conductive material, such as ITO, are deposited on the layer 27 and extend into each of these pits, either by deposition over a mask or by selective etching a continuous layer deposited on the layer 27. Each dot 34 has the form of a rectangular or circular layer extending over its respective dimple 30 and laterally over the immediately adjacent surface of the layer 27 around the dimple, as shown in Fig. 2. Thus, each pixel 10 comprises a regular 4-by-4 matrix of dots 34 (sixteen in total) which, viewed from above in Fig. 2 and as shown in Fig. 3, occupy a major part of the area of the pixel and are uniformly spaced from one another in that area. These dots 34 form the pixel electrodes associated with the carrier plate 22, each of which is actually distributed into sixteen individually energizable sub-electrodes 34 defining a corresponding number of pixel sub-elements CLC which collectively constitute the pixel electrode on the carrier plate 22.

Each 4-by-4 matrix of sub-electrodes together with the overlying portion of a conductor 14 serving as a spaced electrode and the intermediate liquid crystal material form a picture element.

The thin layer of silicon nitride 32 on the bottom of each pit 30 together with the respective directly adjacent surface portions of the conductors 12 and the points 34 form a non-linear two-terminal diode structure electrically connected in series between the conductor 12 and the associated pixel sub-electrode determined by the point 34. The sixteen sub-electrodes of each pixel are therefore connected to the same conductor 12 via a respective diode structure.

Another polyimide alignment layer 35 is deposited on the layer 27 and the dots 34.

The effective electrical circuit configuration of a typical picture element and associated switching elements of the display arrangement of Fig. 2 is illustrated in Fig. 4. For simplicity, only three of the sixteen sub-electrodes and associated diode structures are shown, and any stray capacitances that may occur are neglected.

The MIM diode structures are non-linear two-terminal bidirectional devices and are shown here at 38 for simplicity by oppositely poled diodes. Applying a suitable voltage with the scanning signal and the data signal to the electrodes 12 and 14, respectively, which exceeds the threshold voltage of the diode structures to the row conductor 12 causes the switching of the diode structures 38 and their conducting state under the pooling Frenkel effect so that the voltage is transferred to the dot sub-electrodes 34. This voltage, in cooperation with another voltage signal (the data signal) simultaneously applied to the respective conductor 14, produces the required electro-optical effect in each of the image sub-elements defined by the sub-electrodes 34. Below this threshold voltage, the diode structures 38 are non-conductive. Since the diode structures are bidirectional and substantially symmetrical devices, the polarity of the applied voltages can be reversed in subsequent fields to avoid a DC component acting on the liquid crystal material and causing the resulting electrochemical degradation of the material.

Such a device is relatively simple and inexpensive to manufacture. Furthermore, if one or even several sub-elements of the picture element fail due to defects in the associated diode structures, the picture element can still function adequately to produce an acceptable display effect. Since all of the component layers are substantially transparent, the passage of light through the arrangement in the transmission mode is not significantly hindered by the number of switching elements below the picture element electrodes. The dimensions of the diode structures 38 in Fig. 2 have been deliberately exaggerated and in reality the area occupied by them is small in relation to the picture element area. Therefore, their influence on the light transmission need not be particularly great even if the diode structures 38 are not transparent.

Fig. 5 shows a schematic form of a cross section through a second embodiment of an active matrix liquid crystal display device according to the invention. This embodiment has much in common with the previous embodiment. Therefore, corresponding components are designated by the same reference numerals and will not be described in detail here.

An important difference between the embodiment of Fig. 5 and that of Fig. 2 is that the MIM diode structures are shaped in a different way and their poles spaced from the address conductors 12 are not directly connected to the pixel sub-electrodes, but instead are capacitively coupled to these sub-electrodes.

A thin layer 40 of non-stoichiometric silicon nitride is coated with a thickness of about 0.05 µm (500 Angstroms) on the conductors 12 and on the intermediate surface area of the support plate 22. Silicon oxide may be used instead of silicon nitride. On the surface of this layer 40 is provided another thicker layer 41 of insulating material, such as silicon nitride or polyimide, in which are embedded ITO grains 43 lying directly on the thin layer 40. These grains 43, which are generally spherical or of substantially the same dimension, about 1 µm in diameter, are randomly distributed in the layer 41 at a density which ensures that an appropriate number of grains, for example about 15 to 100, are scattered on the surface of the layer 40 on the conductors 12 at each pixel location.

The grains 43 may be dispensed onto the layer 40 as an alcohol-containing solution. After evaporation of the alcohol, the layer 41 is deposited using silicon nitride to cover and surround the grains separate from the area where they contact the thin silicon nitride layer 40. In another method, where the layer 41 contains polyimide, the grains 43 may be thoroughly mixed with polyimide and the mixture then sprayed over the surface of the layer 40, allowing the grains to settle on the thin layer 40 and contact the layer 40. Both of these deposition techniques offer the advantage of avoiding the need for a photolithographic process and are relatively simple and economical to perform.

While the distribution of grains 43 in layer 41 is more or less random, the proportional amount of grains used in the material of layer 41 provides a sufficient number of grains at each pixel location for the desired purpose, as will be explained below. In the example of Fig. 5, the resulting distribution of grains 43 is such that a plurality of grains, in this particular case five or six, are obtained across the width of each complete conductor 12, substantially uniformly spaced from one another. Of course, the grains are not necessarily aligned as precisely as shown in this schematic. The grains are evenly distributed along the lengths of the conductors 12, and therefore along the other dimension of the pixel. When dealing with an area of the conductors 12 corresponding to a typical pixel area, this is about twenty-five grains 43 in a planar two-dimensional field. Since the distance between the grains cannot be determined exactly, their distribution will generally be such that in each of these regions the resulting field occupies a major part of the region.

Each of the grains 43 overlying a conductor 12 forms, in conjunction with a portion of the thin silicon nitride layer 40 and a surface portion of the electrode 12 directly beneath the layer, an MIM diode structure.

The layer 41 has a thickness greater than the dimension of the grains 43 so that the uppermost surface of the grains is spaced from the surface of the layer 41. On the surface of this layer 41 are deposited discrete electrode layers 45, for example of ITO, which form picture element sub-electrodes. Each picture element is associated with, for example, four such sub-electrodes arranged in a planar 2-by-2 matrix array, although the actual number of sub-electrodes 45 for each picture element can be varied as required. The four sub-electrodes, which have the same dimensions and may be rectangular or circular, collectively form the picture element electrode on the carrier plate 22 and each defines, with a corresponding overlying portion of the electrode 14 and the intermediate liquid crystal material, a picture sub-element. Each sub-electrode 45 overlies a number of, typically between four and nine, diode structures which serve to control the supply of voltages. The sub-electrodes 45 are separated from their associated diode structures by silicon nitride material of layer 41 so that electrical coupling between the diode structures and the sub-electrodes is capacitive.

The sub-electrodes 45 and the intermediate surface areas of the layer 41 are covered by a polyimide alignment layer 35.

The effective electrical circuit configuration of a typical pixel of the pixels of this embodiment is shown in Fig. 7. In this figure, the MIM diode structures are represented by oppositely polarized diodes 47 and the capacitive coupling between the diode structures 47 and the sub-electrodes 45 by the capacitors 48.

It should be noted that, like the previous embodiment, this embodiment offers a high degree of fault tolerance. Since each pixel 10 is associated with a plurality of diode structure switching elements, and each Since the individual sub-electrode 45 is itself associated with a number of diode structures, defects in a number of the switching elements can be tolerated without greatly affecting the operation of the pixels. As with the previous embodiment, the passage of light is not significantly affected when the device is operated in the transmission mode through such large numbers of switching elements below the pixel electrodes, since substantially transparent materials are used to form these switching elements. As above, the switching elements associated with the pixels can be formed small enough that they have little effect on the transmission of light, even if they are not substantially transparent.

This embodiment offers the further advantages over the previous example that changes in the voltage applied to the electrodes of the picture elements are reduced by threshold changes in the individual diode structures using capacitive division by the number of capacitors 48 associated with each sub-electrode 45. The capacitance value of the capacitors 48, which is determined by the size of the grains 43, is small in relation to the capacitance of the picture sub-elements, which is determined by the size of the sub-electrodes 45. Therefore, a relatively large voltage drop occurs across each capacitor 48. For example, if the effective capacitance of the number of capacitors 48 connected in parallel and associated with a picture sub-element is one tenth of the capacitance value of the picture sub-element, a 1 volt change across the diode structures 47 associated with that sub-element causes a variation of only 100 mV across the sub-element.

Operation of the display device is otherwise generally the same as that described with respect to the embodiment of Fig. 2. It is necessary to reverse the polarity of the applied drive voltages after each field in order to reset the pixels, otherwise charge will be fixed in each pixel. Such polarity reversal also serves to prevent electrochemical degradation of the liquid crystal material, as described above.

Fig. 6 shows a schematic cross section through a third embodiment of a display arrangement according to the invention. This embodiment is very similar to the embodiment according to Fig. 5 and can be considered a modification of this embodiment. here again the

For the sake of simplicity, corresponding parts of the arrangement are designated by the same reference numerals.

In this embodiment, the grains 43 are replaced by discrete dots 50 of conductive material such as ITO. The dots are deposited onto the thin silicon nitride layer 40 by evaporation through a mask prior to the deposition of the thicker silicon nitride layer 41, so that the size and spacing of the dots is largely controlled so that they are regularly spaced in relation to the actual picture elements and their centers are substantially identical to those of the actual picture elements. The mask used in depositing the dots 50 can be a shadow mask. The dots 50 formed in this way are substantially uniformly distributed over the surface of the layer 40. The silicon nitride layer 41 is then applied to the dots 50 and again has a thickness greater than the height of the dots 50 so that the upper surfaces of the dots are spaced from the surface of the layer 41. The upper surfaces of the points 50 and the opposite surface parts of the sub-electrodes 45, together with the silicon nitride of the layer 41, which separates them from each other as electric, form capacitors that couple the associated MIM diode structures and sub-electrodes 45.

In this embodiment, each pixel is provided with nine sub-electrodes 45 which collectively form the pixel electrode and are arranged in a planar 3-by-3 matrix array, occupying an area substantially corresponding to the area of the pixel. The dots 50 are distributed such that each sub-electrode is associated with three diode structures.

The electrical circuit configuration of a typical element of the picture elements of this embodiment is generally the same as that of Figure 7, except that there are nine picture sub-elements instead of the four shown, and each sub-element is associated with exactly three diode structures 47 and three series capacitors 48. The operation of the arrangement is generally the same as that of the previous embodiment, with the same advantages being obtained.

Fig. 8 and 10 show modifications of the embodiments of Fig. 2 and 5. As already stated above, identical components are designated with the same reference numerals. In each of these modifications, the Display devices have been modified to minimize capacitive coupling between the picture element sub-electrodes and the underlying portions of their associated address lines while providing a storage capacitor for each picture sub-element.

The storage capacitors are designed for capacitance values that are many times larger than the capacitance of the image sub-elements.

Turning first to the modification of the embodiment of Figure 2 in Figure 8, it can be seen that a further electrically conductive layer 80 is incorporated into the device structure for each pixel. These layers 80 are transparent and are made of, for example, ITO. In terms of the complement of layers 80, silicon nitride layer 27 comprises two separately deposited layers 81 and 82, with the second layer 82 being applied over the first layer 81, sandwiching the layers 80 deposited on the first layer prior to the deposition of layer 82. The layers 80 themselves are formed by depositing a continuous layer of ITO on the first silicon nitride layer 81 and selectively etching this layer to leave the required group of individual layers 80 extending over an area substantially corresponding to their respective pixels 10 and substantially identical to the centers of these pixels 10. Each layer 80 and layers 81 and 82 are formed with openings through which the conductive layers forming sub-electrodes 34 extend for contacting the silicon nitride thin films 32, the layers 80 being slightly spaced from each other and therefore electrically isolated from their associated sub-electrode layers 34 and also from the underlying address lines 12.

The layers 80 of the picture elements 10 in each respective row are electrically isolated from each other. The layers 80 of each picture element in each respective column are electrically connected to each other by integral bridges, not shown in Fig. 8, remaining after selectively etching the continuous conductive layer to form the layers 80, and each connected column of layers 80 is connected to its associated column conductor 14 by means not visible in Fig. 8.

The effective circuit configuration for a typical pixel of the arrangement of Fig. 8 is illustrated schematically in Fig. 9. In comparison of the 9 with that of FIG. 4, it can be seen that portions of the layers 80 in conjunction with their respective overlying sub-electrodes 34 and the intermediate insulating layer 80 form a capacitance Cs in parallel across each image sub-element CLC. The capacitances Cs serve in a known manner as storage capacitors during operation of the device.

In Fig. 10, the illustrated display arrangement is a variation of the embodiment of Fig. 5 with changes similar to those described with reference to Fig. 8. In particular, additional transparent and electrically conductive layers are disposed between the pixel sub-electrodes 45 and the underlying conductor 12. In this case, however, the additional conductive layer of each pixel is configured to define physically separate portions with the reference numeral 90 which generally correspond in dimension to one another and will be registered with a respective pixel sub-electrode 45. However, the layer portions 90 associated with the sub-electrodes 45 of each pixel are still electrically connected to one another, although these connections are not visible in Fig. 10.

The layer portions 90 and their connections are formed on the surface of the silicon nitride layer 41 by selectively etching a deposited continuous layer and spaced from the sub-electrodes 45 by another silicon nitride layer 91 extending continuously across layers 41 and 90. The layer portions 90, which generally correspond in shape to their associated pixel sub-electrode 45, are electrically insulated from their associated overlying sub-electrode 45 and underlying address lines 12. The layers 90 and 91 are provided with registered openings through which the sub-electrodes 45 extend to contact the upper surface of the layer 41 for capacitive coupling to the diode structures beneath this extension in a manner similar to that of the embodiment in Fig. 5. The extensions of the sub-electrodes 45 are insulated from the layer portions 90 by material of the layer 91. In comparison to the devices according to Fig. 10 and 5, it can be seen that in the configuration according to Fig. 10, the picture element sub-electrodes 45 are no longer completely planar, but surround the diode structure with their surrounding parts and are further away from the conductor 12 in order to apply the additional conductive layer 90 and the required further insulating layer 91.

The layers 90 of adjacent picture elements in the same row are electrically isolated from each other.

Again, the picture elements of each respective column of layers 90 assigned to picture elements are electrically connected to one another (via column-wise bridges, not visible in Fig. 10, which remain after etching the continuous conductive layer) and to the respective column conductor 14.

The effective electrical circuit configuration of a typical picture element of the display device according to Fig. 10 is shown in simple form in Fig. 11. Components corresponding to those in Fig. 7 are designated by the same reference numerals. As can be seen from this figure, each picture sub-element CLC is associated with a storage capacitance CS in parallel, this storage capacitance being formed by the layer part 90 and the opposite part of the sub-electrode 45 with insulating material of the layer 91 in between.

The embodiment of Fig. 12 is very similar to that of Fig. 8 with the exception, as in the embodiment of Fig. 10, that the pixel sub-electrodes, again designated 34 for simplicity, are capacitively coupled to their associated switching elements. In this example, each pixel 10 contains a 2-by-2 array of pixel sub-elements, each capacitively coupled to a switching element. Components of the arrangement which generally correspond to those of Fig. 8 are designated by the same reference numerals.

As previously described, a continuous conductive layer 80 is disposed between the sub-electrodes 34 of each pixel and its underlying address line 12 and separated therefrom by layers 82 and 81, respectively. Layer 80 is provided with openings which overlie the pits 30 formed in layer 81. Thin films of non-stoichiometric silicon nitride 32 are deposited at the bottom of these pits. However, instead of the sub-electrodes 34 extending downward into the pits 30 to contact the thin films 32, as before, they extend only partially into the pits in this embodiment. When deposited on the upper surface of the thin films 32 in the pits 30, respective conductive layers 95 are formed simultaneously with the layers 80 and thus of the same material as the layers 80 before the deposition of layer 82. The layer 82 is applied both over the layers 80 and the layers 95 in the pits 30, so that in the following Deposit the conductive layer forming the lower electrode 34 extends downwards over the layer 82 in the region of the pits 30 and is applied by the layer 82 at a distance from the conductive layers 95.

The layers 95 together with their associated underlying films 32 and surface portions of the address lines 12 form MIM diode structures as previously described, with one diode structure being provided for each sub-electrode and therefore each sub-pixel of each pixel. These diode structures are capacitively coupled to their respective sub-electrodes 34.

As previously mentioned, the layers 80 of the pixels in a column are electrically connected column by column via bridges not shown in Fig. 12, but are electrically isolated from the layers of adjacent pixels in the same row. Each connected column of layers 80 is connected to the opposite column conductor 14.

The effective electrical configuration of a typical pixel is similar to that shown in Fig. 9, which shows a typical pixel of the embodiment of Fig. 8, except that a coupling capacitance similar to that shown at 48 in Fig. 11 is provided in series between each diode structure switching element 38 and its associated pixel sub-electrode 34. The layers 80, in conjunction with the overlying sub-electrodes 34 and intervening insulating material of the layer 82, form a storage capacitor Cs for each pixel sub-element.

With respect to the embodiments of Figures 8, 10 and 12, the conductive layers 80 and 90 may be electrically connected to a source of fixed potential instead of to the associated column conductor 14, the polarity of this potential being reversed for alternating fields.

The embodiment described with reference to Fig. 7 can be modified in a similar manner to that described with reference to Fig. 10 for the embodiment of Fig. 5 to accommodate storage capacitors by including a transparent conductive layer between each sub-electrode 45 and the underlying address conductor 12.

In all of these embodiments with storage capacitors, the additional conductive layers 80 and 90 provided serve to additionally shield the picture element sub-electrodes 34 and 45 from their underlying address lines 12 and thus minimize any capacitive coupling between the sub-electrodes and the address lines that might otherwise occur.

Although in Figs. 8 and 12 the additional conductive layer 80 associated with each pixel is shown as continuous, they may instead be physically defined as separate parts for each sub-electrode while still remaining electrically connected to one another in the same manner as described with reference to Fig. 10. Conversely, the individual defined layer parts 90 of the Fig. 10 embodiment could be formed as a continuous layer that is substantially coextensive with the respective pixel region but still provided with openings through which the sub-electrode layers 45 extend to the switching elements in a manner similar to that of Figs. 8 and 12.

All of the above-described embodiments and modifications offer the advantage over known display arrangements that the space between the image elements can be minimized, since the address lines are arranged on a carrier plate and the associated switching elements are arranged below the image elements, thereby deviating from the known arrangements in which the switching elements are arranged on the carrier plate to the side of the image elements.

Claims (16)

1. Liquid crystal display arrangement with a pair of spaced-apart carrier plates (20, 22), with liquid crystal material (24) between the plates, with a number of picture elements (10) in a matrix field, which are defined by mutually opposite electrodes on the carrier plates, and with a number of switching elements (16) on one (22) of the carrier plates and in series connection between address lines (12) on this plate and the picture elements (10), in which the picture element electrodes on the one carrier plate (22) overlie both the switching elements and the associated address lines on this one carrier plate, the switching elements being arranged between these picture element electrodes and address lines, characterized in that each of the picture element electrodes on the one carrier plate (22) contains a number of discrete sub-electrodes (34, 45), each of which is associated with at least one switching element (38, 47), the switching elements associated with a picture element electrode Switching elements are connected in parallel to each other with the corresponding address line (12). 2. Liquid crystal display device according to claim 1, characterized in that the switching elements are non-linear two-pole bidirectional elements. 3. Liquid crystal display device according to claim 2, characterized in that the switching elements contain diode structures. 4. Liquid crystal display arrangement according to claim 3, characterized in that the diode structures each contain a thin layer (32, 40) of silicon nitride material in a sandwich construction between two electrically conductive layers (34, 43; 12) of indium tin oxide material, the two electrically conductive layers forming respective poles of the diode structure. 5. Liquid crystal display arrangement according to one or more of claims 2 to 4, characterized in that the switching elements assigned to the picture elements are arranged in a substantially planar field below the picture element electrodes on the one carrier plate. 6. Liquid crystal display device according to claim 5, characterized in that the substantially planar field of switching elements associated with each picture element occupies an area which corresponds to at least a major part of the area of the picture element electrode on the one carrier plate. 7. A liquid crystal display device according to claim 6, characterized in that the switching elements of the substantially planar array are arranged substantially uniformly and spaced apart from one another with respect to the region of the picture element electrode. 8. Liquid crystal display device according to one or more of the preceding claims 2 to 7, characterized in that a conductive layer (34) which represents one pole of the two-pole switching elements also serves as the associated picture element sub-electrode. 9. Liquid crystal display arrangement according to one or more of claims 2 to 7, characterized in that a conductive component (43, 50) which represents one pole of the two-pole switching elements is separated from its associated pixel sub-electrode (45) and is spaced therefrom by insulating material (41) in order to form a capacitive coupling between the switching element and its associated pixel sub-electrode on the one carrier plate (22). 10. Liquid crystal display device according to claim 9, characterized in that the conductive components which form the one poles of the switching elements contain conductive grains (43) which are interspersed in an insulator matrix material (41). 11. A liquid crystal display device according to claim 9, characterized in that the conductive components forming the one poles of the switching elements comprise discrete points (50) of deposited conductive material. 12. Liquid crystal display arrangement according to one or more of the preceding claims 8 to 11, characterized in that the other pole of each of the two-pole switching elements is formed by a respective part of its associated address line (12) on the one carrier plate (22), wherein the one pole (34) of the switching elements is located vertically to the carrier plate by a thin insulating layer (32) at a distance from its associated address line (12). 13. Liquid crystal display device according to one or more of the Claims 1 to 12, characterized in that the display arrangement contains storage capacitors assigned to the picture elements. 14. Liquid crystal display device according to claim 13, characterized in that each picture sub-element determined by a respective picture element sub-electrode is linked to a respective storage capacitor. 15. Liquid crystal display device according to claim 14, characterized in that the storage capacitors are partially formed by at least one conductive layer (80, 90) on the one carrier plate, which underlies the picture element sub-electrodes (34, 45) on the one carrier plate and is electrically insulated therefrom. 16. A liquid crystal display device according to claim 15, characterized in that the at least one conductive layer (80, 90) is arranged between the pixel sub-electrodes and the associated underlying address lines to substantially shield the pixel sub-electrodes from the address lines and to minimize capacitive coupling between them.